Cryptosporidium parvum is an AIDS-OI pathogen and Category B agent that can cause severe watery diarrhea in humans and animals. This parasite can cause prolonged, life threatening infection in immunocompromised individuals, and is responsible for a substantial degree of morbidity and mortality in AIDS patients. Also because the infectious oocysts are highly resistant to chemical stresses, C. parvum is a significant water- and food-borne pathogen and one of the category B agent in the NIH biodefense program. Currently, no drug is FDA-approved to treat cryptosporidiosis in immunocompromised patients, although a single drug (nitazoxanide) is approved for use in immunocompetent patients in the United States. Therefore, there is an urgent need to develop new anti-Cryptosporidium drugs, particularly new drugs for AIDS patients. Encouraged by the recent development in repurposing of existing drugs for potential new indications, as well as based on our unexpected observations that a marked drug could strongly inhibit the growth of Cryptosporidium in vitro, we plan to extend our discovery by screening all existing drugs for their potential ant-cryptosporidia activities in vitro and in vivo. Briefly, we will take advantage of established models and assays t achieve the following two aims: Aim 1. To identify potential anti-cryptosporidial compounds from known drugs in vitro using our well- developed and improved qRT-PCR assay. Aim 2. To determine anti-cryptosporidial efficacy of select drugs in vivo using acute and chronic mouse models of cryptosporidial infection. The majority of the compounds being screened are currently or previously marketed drugs. The proposed experiments examine the direct effects of drugs on the parasite growth in vitro and in vivo. Therefore, if satisfactory anti-cryptosporidial activitie in vitro and in vivo are observed among any of the FDS-approved drugs, these effective drugs can be rapidly moved to clinical trials and repurposed to become new drugs to treat cryptosporidial infection in humans, thus accelerating the drug discovery and development against Cryptosporidium. Furthermore, the project will nonetheless produce a large amount of data that profiles the activities of nearly all existing and abandoned drugs. The invaluable data can be used to analyze the structure-activity relationship (SAR) against Cryptosporidium to guide future drug development.